1. Field of the Invention
The present invention relates to digital communications test systems, and in particular, to test systems for testing multiple input, multiple output (“MIMO”) systems.
2. Related Art
Testing of MIMO systems with multiple channels using conventional test equipment presents a number of challenges. Generally, multiple test instruments are required and must be synchronized in frequency and time to produce and analyze the MIMO signals of a device under test (DUT). One example of such a system is the “IQn×n” test system designed, manufactured and sold by LitePoint Corporation of Sunnyvale, Calif. Such a system uses multiple vector signal analyzers (VSAs) and vector signal generators (VSGs). However, one disadvantage of such a system is the additional cost of multiple VSAs and VSGs, which often makes use of such systems cost prohibitive for manufacturing in which multiple such systems are required. Accordingly, to minimize costs during manufacturing tests, solutions have been developed allowing the test of a MIMO DUT using a single VSA and single VSG. Such solutions focus on testing the DUT transmitter using a single VSA. Examples include composite error vector magnitude (EVM) analysis or switched EVM analysis. One example of a composite EVM test system and method can be found in U.S. patent application Ser. No. 11/533,971, filed Sep. 21, 2006, and entitled “Apparatus and Method for Simultaneous Testing of Multiple Orthogonal Frequency Division Multiplexed Transmitters With Single Vector Signal Analyzer” (the disclosure of which is incorporated herein by reference).
Alternatively, switched EVM testing can be performed in which the output of each transmitter can be captured individually by switching the input to the VSA between the different DUT transmitters. The resulting captured data are combined and analyzed as if the captured data was from the same packet.
Such testing techniques do allow testing of the MIMO functionality of the DUT, although perhaps not fully according to the specification established by the Institute of Electrical and Electronics Engineers (IEEE). However, such testing does operate the DUT in a challenging mode of operation by transmitting multiple streams of data through the different DUT transmitters. Both testing techniques can encounter limitations for testing MIMO systems, i.e., switched EVM testing can be problematic when testing DUT transmitters employing spatial diversity expansion. Similarly, composite EVM testing can be problematic for measuring single data stream operation transmitted via multiple transmitters, as well as transmitter isolation. Further, both testing techniques require known data in the transmitted data packets.
What is also lacking is an effective testing technique for testing the multiple receivers in a MIMO DUT using MIMO signal waveforms. This limitation exists due to the use of a single VSG which allows only one data stream to be generated, so the same data stream will be presented to all of the DUT receivers. One option is to test MIMO operation by verifying the maximum ratio combining (MRC) capability of a DUT using a single data stream by comparing the difference between the sensitivity when the signal is received by a single receiver and the sensitivity when the same signal is received by all DUT receivers. (As is well known in the art, MRC testing combines the signals from multiple spatial diversity branches, with each branch signal multiplied by a weight factor that is proportional to the signal magnitude i.e., stronger signals are further amplified while weaker signals are attenuated.) This tests the capability of the DUT to process parallel receive data streams and, from that, improve the sensitivity of the DUT (the theoretical improvement is 10*log 10N, where N is the number of DUT receivers). However, to perform a real MRC test, it is generally necessary to compare sensitivity points between the various receiver inputs to verify true MIMO performance.
Determining this sensitivity point is generally a long process, requiring more time than a simple packet error rate (PER) test, although some improved testing techniques have been developed such as U.S. patent application Ser. No. 10/908,946, filed Jun. 1, 2005, and entitled “Method for Measuring Sensitivity of Data Packet Signal Receiver” (the disclosure of which is incorporated herein by reference), and U.S. patent application Ser. No. 11/161,692, filed Aug. 12, 2005, and entitled “Method for Measuring Multiple Parameters of a Signal Transmitted By a Signal Generator” (the disclosure of which is incorporated herein by reference). Further, it is possible that a MRC test using a single data stream may not test the true MIMO capability of the DUT, since it cannot exercise the true MIMO multi-stream processing of the DUT, and thus not verify performance at the highest data rates. Such a MRC test generally ensures only that the MRC signal processing is functional and that there is no coupling between the different DUT receiver signal paths that might prevent MIMO operation with multiple data streams. Accordingly, it is generally assumed that the signal processing inside the DUT will work (as it was supposed to have been tested by the chip manufacturer).
One alternative technique often used is including “a golden unit” i.e., a manufactured unit that has been fully, and often manually, tested as part of the test system to ensure operation at the maximum data rate. However, this is not ideal as a manufacturing solution, since accuracy, reliability and repeatability can vary, often significantly.
Until recently, these limitations on production testing have been more or less acceptable since the data rates needed in typical uses has been much lower than the data rates of which MIMO systems are capable. For example, when a typical user is accessing the Internet, data rates are typically limited to 10 megabits per second (mbps) or less from the Internet service provider (ISP). Accordingly, a MIMO system capable of more than 100 mbps will not be fully exercised. Hence, if the system is not fully operational, e.g., the system is not capable of receiving multiple streams of data at its maximum data rate, the user will not likely notice as the available data rates of 10 mbps can easily be served via a single data stream.
However, new devices and applications are using the higher bandwidths provided by MIMO. For example, video streaming can produce peak data rates greater than 100 mbps, as can some newer wireless data storage systems which require the highest possible data rate so as to perform a system data backup in the shortest time possible.